A murine gene controlling resistance to flavivirus-induced encephalitis was first identified in 1936. The resistance phenotype was found to be inherited as an autosomal Mendelian dominant trait. Subsequent studies have indicated that the product of the flavivirus resistance gene specifically affects the level of replication of flavivirus, but not that of unrelated viruses. The means by which the resistance gene product produces a flavivirus specific effect is currently not known. One possibility is that the gene product affects the functioning of the flavivirus polymerase. Recent studies have indicated that resistant cells are inefficient in synthesizing flavivirus genome RNA and able to preferentially synthesize WNV DI RNAs. The present proposal describes a basic study of the characteristics of the flavivirus polymerase and an investigation of the possible dependence of viral polymerase activity and/or template specificity on a host cell protein. Studies will be conducted to first identify, isolate, and characterize the viral protein(s) responsible for flavivirus polymerase activity and subsequently, to study the dependence of flavivirus polymerase activity on a cellular protein. The flavivirus used for these studies will be West Nile Virus (MNV). A replication-efficient mutant of WNV has been isolated. In contrast to what is observed with the parental WNV, this mutant can efficiently synthesize genome RNA in resistant cells and is insensitive to interference by WNV DI RNAs. The characteristics of the mutant polymerase protein(s) will be compared with those of the polymerase of the parental WNV. Also, the genome of the mutant virus will be used for comparative analyses of 3 feet terminal sequences. This study of the dependence of flavivirus polymerase activity and/or template specificity on a cellular protein will increase our knowledge of basic cell biology as well as provide information on the mode of replication of a medically important group of viruses.